A Push-to-Talk (PTT) mode of communication is designed to minimize the delays associated with communicating between communication devices, so as to provide near-instantaneous connectivity. Typically operating in a half duplex channel configuration, which allows one communication device to transmit at a time, a communication device operating in a PTT mode may transmit to a single receiving terminal, or to a group of receiving communication terminals.
A simple example of a known PTT system is a set of two walkie-talkies. In this PTT system the walkie-talkie transmits at a particular frequency to another walkie-talkie, which is tuned to the same frequency. When the walkie-talkie transmit or talk button is depressed, the user may speak and transmit over his walkie-talkie. When the talk button is released the walkie-talkie does not transmit, but may receive transmissions from the other walkie-talkie. Because there is no network involved here, there are no time delays associated with the initiation and completion of a communication link. The significant drawback of such a system, however, is the limited operational range; normally the system does not operate when the distance between terminals exceeds about a mile.
A trunked communication system such as a cellular network may be used to support PTT mode communications. A typical PTT system 100 implemented over a cellular network, as known in the art, is depicted in FIG. 1, which shows a first communication device CD 110 transmitting to a second communication device CD 115. This PTT call is transmitted wirelessly to a first Base Station System (BSS) 120. From the BSS 120, the call request is routed to a first Mobile Switching Center (MSC) 130, which is associated with the BSS 120 serving the CD 110. The MSC 130 then routes the call to a PTT server 140, which is responsible for contacting the intended recipient and establishing floor control for the communications channel. When the CD 110 transmits over the assigned traffic channel, the PTT server 140 routes the call via a second MSC 150 and a second BSS 160 to the intended receiving communication device CD 115.
PTT communication networks, as currently implemented, are expensive and inefficient for conducting PTT calls between communications devices. The inclusion of a PTT server necessary for floor control in the known networks, introduces several time delays into the overall system performance. The user of the initiating communication device must wait for a PTT request to be sent to the PTT server 140, and wait for a responding acknowledgement, typically a chirp, prior to beginning to speak. Thus, there is a delay associated with contacting and receiving acknowledgement from the PTT server 140 before the user can begin to speak a message.
In addition to introducing time delays in the communications link, a PTT server necessitates the use of additional network resources. In a trunked communications system, channels are allocated and shared among multiple users according to need and availability. In the PTT network described above, a PTT server is an integral component of the communications network regardless of the number of targeted recipients. The inclusion of the PTT server necessitates the formation of additional trunk lines and allocation of network resources for establishing a communication connection between two communication devices, which could be performed by a standard cellular network without assistance from a dedicated PTT server.
A third disadvantage of a PTT server is that the receiving party may not cut in and speak while the sending party is speaking. The PTT server exercises floor control over the channel so that one communication device may not transmit over the PTT channel while another communication device is transmitting.